Alkylation is a process whereby an isoparaffinic hydrocarbon such as isobutane is reacted with an olefin (such as propylene, butylene and/or pentene) to form a high octane hydrocarbon stream known as alkylate that boils in the gasoline boiling range of from about 100.degree. to about 430.degree. F. There are two primary catalysts typically employed to promote this reaction, hydrofluoric acid (HF) and sulfuric acid.
The HF alkylation process operates at essentially atmospheric temperature (about 80.degree. to about 120.degree. F.) and ordinarily does not require a refrigeration system. However, in some instances, it may be feasible to operate the process at lower temperatures as would require refrigeration.
The sulfuric acid alkylation process is used in numerous refineries and operates in the range of from about 35.degree. to about 60.degree. F., and more specifically in the 40.degree. to 50.degree. F. range, to prevent the formation of undesirable products. Thus, refrigeration is necessary in order to chill the inlet streams and to remove exothermic heat of reaction.
At least four types of reactor systems typically are employed in commercial alkylation facilities, including the time-tank, the cascade, the effluent refrigeration, and the closed cycle refrigeration system utilizing the effluent refrigeration system reactor. Each of these will be described later herein.
In almost every instance, the reaction temperature is maintained at 40.degree. F. to 50.degree. F. by employing a system utilizing the vapor compression refrigeration cycle. Major components include: evaporation, compression, condensation, and recycle of refrigerant. In the closed cycle systems such as time-tank and the closed cycle effluent reactor, ammonia or propane is used predominantly as the refrigerant. In the open systems, isobutane vapors created by the process are the refrigerant. These vapors are predominantly isobutane, but include equilibrium concentrations of ethane, propane, normal butane, pentanes, and minor quantities of heavier hydrocarbons vaporized from the reaction mixture.
Because process conditions vary so greatly from unit to unit and between geographic areas, one cannot generalize on the energy requirements to produce a given quantity of alkylate product. However, by way of example, a conventional effluent refrigeration unit designed to produce 8,000 B/D of alkylate may require a 5500 to 6000 horsepower refrigerant compressor.
Obviously, substantial energy in the form of electricity or high pressure steam, etc. is necessary to carry out compression as described above. The present invention is directed to a method whereby these energy costs may be reduced substantially, resulting in significant economic benefit. By using sources of heat in a refinery that might otherwise be lost to air and water from streams being cooled, for example, below about 300.degree. F., this "waste" heat is used in the present invention to create refrigeration for the alkylation unit through the use of absorption refrigeration. Sources of low (temperature) level heat that may be employed in the system of the present invention include low pressure steam, furnace stacks, hot liquid and vapor streams, and the like. While the system of the present invention may dispense with the use of a compressor in an alkylation refrigeration system, the preferred embodiment herein contemplates the use of a compressor but reducing the compression ratio from the typical 5 to 8:1.0 level to about 2.0 to 2.5:1.0 with consequent substantial reduction in refrigeration expense.
It is, therefore, an object of the present invention to provide an improved alkylation process whereby vaporous excess and unreacted isoparaffinic hydrocarbons from the reactor are cooled and condensed for recycle to the reactor by means of absorption refrigeration.
Another object of the present invention is to provide such an improved alkylation process whereby cooling and condensing of the vaporous unreacted isoparaffinic hydrocarbons may be effected by either direct or indirect heat exchange in the absorption refrigeration step.
A still further object of the present invention is to provide an improvement in the catalytic alkylation of an isoparaffinic hydrocarbon such as isobutane with an olefin wherein the olefin is contacted with a molar excess of isobutane in the presence of a catalyst in a reactor, the reactor effluent including at least alkylate and isobutane. From the reactor effluent is separated a substantially liquid stream containing alkylate and isobutane and a substantially vaporous stream containing predominantly isobutane. Systems presently utilizing closed cycle refrigeration systems will be converted to this type refrigeration. The substantially vaporous stream is admixed with an absorbent and condensed to substantially a liquid followed by distillation of the condensed liquid whereby isobutane vapor is separated as distillate from the absorbent. The isobutane distillate is returned to the reactor to maintain an excess of the isoparaffinic hydrocarbon and for cooling purposes.
Yet another object of the present invention is the provision of such an improved alkylation process wherein, prior to admixing the substantially vaporous stream from the reactor with the absorbent, the stream is compressed to a higher pressure.
Still other objects, features and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention, given for the purpose of disclosure and taken in conjunction with the accompanying drawings.